(1) Field of the Invention
The present invention relates to input devices for electronics and, more particularly, to a touch sensitive input panel or display with small form factor especially suited for use in cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, or any other device that uses touch sensitive displays or panels.
(2) Description of Prior Art
Touch screens are being deployed in an increasing number of products using an array of several types of technology. Market analysts predict that in the mobile telephony market, touch screens will increase from less then 10% penetration to more then 50% penetration by 2010, assuming the cost of these touch screens are reduced to a low enough level. It is possible that resistive based touch screens can support low enough prices, but it is certain that force sensing resistor based touch screen can be produced at low enough cost to support this type of market projections.
As consumer products continually decrease in size and increase in user interface complexity and display advancements, the demand for inexpensive, low-profile and precise touch screens is increasing. Indeed, when used in a smaller electronics device the sensor must also be thin, i.e., less than about 1 mm thickness, yet be robust and durable.
In today's electronic industry the manufacturer of an electronic device utilizing a pressure sensitive touch sensitive display solution will look to their display supplier and solution providers for new low cost highly functional touch screens.
There are several types of technologies used in implementing touch sensitive screens that can detect the application of fingers and other passive objects. For example, resistive pads comprise two conductive plates pressed together. The disadvantage of a resistive pad is that the resistive membrane material will wear out, initially resulting in further reduced clarity followed by dead spots. In addition, the production yield is typically rather poor and the technology has a few disadvantages such as a fixed (non-user adjustable) actuation force and the light throughput through the resistive membranes is typically only around 70% to 75%, reducing display visibility.
Capacitive touchpads operate by measuring the capacitance of the passive object to ground, or by measuring the alteration of the trans-capacitance between different sensors.
An example of a capacitive touchpad is described in U.S. Pat. No. 5,495,077 to Miller.
Capacitive pads are relatively expensive to manufacture compared to resistive, and can only detect objects with sufficient capacitance. Small objects, such as the end of a regular stylus or pen, do not have enough capacitance to ground or trans-capacitance to be detected by a capacitive touchpad. Moreover, the actuation force is predetermined and may be as low as 0 grams force, in which case the touch screen may register a touch even before the user's finger touches the screen. This often leads to difficulties in implementing certain end-user features, such as handwriting recognition.
Surface acoustic wave devices operate by emitting sound along the surface of the pad and measuring the interaction of the passive object with the sound. These devices work well, but are generally much too expensive for general applications.
Infra red light based displays work in a similar fashion, but this technology typically adds a large size and a high cost increase.
Finally, there are devices that use force sensors to measure the location and magnitude of the force exerted by the passive object on the touchpad. A force sensitive touchpad will sense force applied by any sort of passive object, regardless of the electrical conductivity or composition of the object. Such devices were originally described in U.S. Pat. No. 3,657,475 to Peronneau et al. and U.S. Pat. No. 4,121,049 to Roeber. These devices measure the forces transmitted by the touchpad to a fixed frame at multiple points e.g., at the corners of the pad. Roeber '049 discloses a mathematical formula for deriving the position and magnitude of the force applied by a passive object from the forces measured at the multiple points.
As another example, U.S. Pat. No. 4,511,760 to Garwin et al. issued Apr. 16, 1985 shows a force sensing data input device responding to the release of pressure force. The input surface is provided with a transparent faceplate mounted on force-sensing piezoelectric transducers. Preferably, four piezoelectric transducers are provided, one at each corner of a rectangular opening formed in the frame. To determine the point of application of force on the input surface, the outputs of the four transducers are first summed. To constitute a valid data entry attempt, the sum must exceed a first threshold while the user is pushing on the input surface. When the user releases his finger, a peak of the sum is detected, which is of opposite polarity from the polarity of the sum for the pushing direction. The individual outputs of the four sensors at the time that the peak of the sum occurs are used to calculate the point of application of the force.
United States Patent Application 20030085882 by Lu published May 8, 2003 shows a touch pad device having a support layer with a plurality of strain gauges in a matrix configuration. A touch layer is disposed on top of the strain gauge matrix, the touch layer being joined to the top of the strain gauge matrix. Sensor wires connect the strain gauges to a processor which is programmed with an algorithm to measure the location and pressure of simultaneous, multiple touches.
United States Patent Applications 20040108995 and 20040021643 both by Hoshino et al. show a display unit with touch panel mounted above a display via four differentially-mounted sensors. The pressure sensors detect force with which a pointing device such as a finger pushes the panel surface, in real time. The force P with which the pointing device such as a finger pushes the panel surface is found from the following equation irrespective of the pointing position: P=a+b+c+d−a0+b0+c0+d0, which equation detects dragging of a cursor.
United States Patent Application 20050156901 by Ma et al. issued Jul. 21, 2005 shows a touch screen display system with a display screen and overlying touch surface. An imaging system determines an angular position on the touch surface of the object coming in contact with the touch surface.
United States Patent Application 20060119589 by Rosenberg shows a haptic feedback feature for touchpads and other touch controls in which at least one actuator is coupled to the touch input device and outputs a force to provide a haptic sensation to the user contacting the touch surface. Output haptic sensations on the touch input device can include pulses, vibrations, and spatial textures, and the compliant suspension amplifies the haptic feedback.
United States Patent Application 20060016272 by Chang published Jan. 26, 2006 shows a thin film touch pad with opposed sensor elements that generate an electrical signal that is proportional to both the applied pressure and the surface area at the location of the applied pressure. As a result of the complementary orientation and overlapping for these sensor elements, the first and second sensor elements generate an asymmetric pair of signals that uniquely define the applied pressure by position and magnitude.
U.S. Pat. No. 6,879,318 by Chan et al. issued Apr. 12, 2005 shows a touch screen mounting assembly for a liquid crystal display panel LCD including a bottom frame, a backlight panel seated in the frame and that has a plurality of pressure-sensitive transducers mounted thereon, a liquid crystal display panel, and a top frame for exerting pressure when mounted to the bottom frame such that a plurality of compressible springs biases the LCD panel towards the bottom frame when touched or contacted by a user. The claims require the bottom and top frame assembly with backlight panel mounted therein on springs, and an overlying LCD panel.
Despite the availability of the existing sensing technologies mentioned herein, the prior art has been unable to provide a low-cost sensor assembly/solution having sufficient sensitivity, surface robustness, accuracy and form factor. Therefore, there is significant industrial applicability in the present invention which provides a force sensing technology which overcomes some of the deficiencies of the prior art.
A commercially viable force-based touch sensor for use with consumer equipment, such as computers, must be both inexpensive and precise. The precision required of such a device is the capability to sense both fingers and pens over a pressure range from about 1 gram to 500 grams or more, with a typical positional precision of +/−1 mm and a resolution of 400 dpi or more. When used in a smaller electronics device the sensor must also be thin, typically less than about 1 mm, and should also be capable of modular assembly for more-or-less “snap-in” construction. The force must in addition to be small and low cost, also be very durable. It needs to allow for millions of press-depress cycles as well as allow for high spikes in the applied force, such as if a device is dropped onto the floor, as well as allow for a wide temperature range, as wide as −40 C to +80 C, at least for storage.
FIG. 1 is a high level representation an electronic device 1, such as a PDA or a cellular phone, having a touch screen assembly 2. One skilled in the art should understand that the touch screen assembly 2 may be incorporated in cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, or any other device that uses touch sensitive displays or panels.
The touch screen assembly 2 employs a touch surface comprising a pressure sensitive lens (PSL) 3 overtop (and preferably bonded to) an underlying LCD or OLED module 5. The PSL 3 covers the LCD/OLED module 5 and may additionally cover static keys on the keypad 4. It is noteworthy that the touch sensitivity area can be extended beyond the display module 5 display area. For example, in the example of FIG. 1 the PSL 3 also extends over a static printed keypad area 4. Regardless of whether a user presses a key in the static keypad area 4 or some portion of the LCD/OLED module 5 area, exactly the same behavior is triggered. The exact “touch-coordinate” is calculated, the touch coordinate is interpreted, and proper control signal(s) are generated. If, for example, the user presses a Left-Arrow Command key, the corresponding left arrow command is generated. As will be described, the touch screen assembly 2 may optionally be equipped with a haptic response generator 12 along with the sensors 7, such as a piezo element or a magnetic inductive coil. In this case whenever the PSL 3 is depressed a short vibration burst is generated by the haptic element 12 and the user can feel as if the “key” was pressed.
It is known to employ a mechanical differential-pressure touch screen system that uses a plurality of force sensors. For example, FIG. 1 is a high level representation an electronic device 1, such as a PDA or a cellular phone, having a touch screen assembly 2.
LCD/OLED module 5 has a plurality (such as, for example, four) differentially-mounted sensors 7 beneath it all connected to the electronic device 1 processor. This way, when a user touches the PSL 3, the touch pressure is transmitted through the LCD/OLED module 5 into the sensors 7 where it is registered, processed, the exact “touch-coordinate” is calculated, and the touch coordinate is interpreted and proper control signal(s) are generated. Within this core context, two basic mechanical embodiments may be used. In one embodiment the sensors are mounted beneath the display module itself. Most conventional display screens (LCD or otherwise) are reinforced with a bonded protective lens. This lens is typically a 0.70 mm to 1.2 mm treated glass, protecting the LCD against cracks, scratching and also providing anti-glare coating. The existing glass lens serves as the primary touch surface, and the force imparted to the primary touch surface is transmitted through the display module and is detected by the differentially-mounted pressure sensors beneath the display module. Alternately, a separate free-floating lens may be used overtop the display module (independently suspended there over). The free floating lens straddles the display module and bears directly against the differentially-mounted pressure sensors.
Examples of both configurations are disclosed in International application no PCT/US2007/019606 filed 7 Sep. 2007. The touch sensitive lens may have some form of mechanical suspension, such as metal beams that suspend the lens and allow for a minor but unrestricted motion in the z-plane with a minimum or no motion in the xy-plane. There are typically four piezo-resistive force sensors in each corner that the floating lens rests on. In order to optimize performance, there may be a pre-loading of the lens through gasket or springs, that presses the lens down onto the force sensors with a preload weight typically greater then the weight of the lens itself. The piezo-resistive sensors are electrically connected via an amplifier to an analog to digital converter (ADC), typically one ADC with enough channels to support the number of sensors. The ADC is then connected via a digital bus, such as 12C or SPI to a microcontroller or an application processor running positioning determination software. The software triangulates the force readings from the force sensors to determine the actual coordinate where the touch force was applied. The software transfers the calculated coordinates to the device's operating system.
Presently most commercial force sensors use piezoresistive materials to detect applied force. While most commercially available piezoresistive force sensors are highly accurate, they typically are not very durable and are large and expensive with only a few available component suppliers globally. There are alternative force sensors available, such as “force sensitive resistors” or FSR's, which are smaller, lower in cost and more readily available. Examples of FSRs are shown in U.S. Pat. Nos. 4,739,299, 4,489,302, 4,451,714, 4,315,238, 4,314,228, 4,314,227 and 4,306,480. A benefit of using integrated FSR sensors is that the voltage output is typically ten times higher then the voltage output for a piezo-resistive force sensor. This higher voltage output eliminates the need for additional analog signal amplification, thereby further reducing both required board space as well as component costs. The mechanical design is further simplified by use of the FSR sensor since these sensors do not need to be protected against overpressure, whereas a typical piezo resistive sensor does. Unfortunately FSR sensors have a much narrower range of sensitivity. In addition, there are also new, not yet commercialized, force sensor materials that are based on nano technology. The early indications from activities in this area are that the nano technology based sensors will be similar to FSR sensors in terms of low per unit cost, small size, yet will require similar added performance compensation and error correction.
Two embodiments of FSR sensors are represented in FIG. 2. These sensors are typically made up of two plans of conductive materials 24 in sensor 20 or conductive traces in sensor 21 that are “connected” through FSR material. The characteristics of the FSR material are that it remains non-conductive until a force is applied. When a force is applied, the resistance in the material decreases as the applied force increases.
An example of such resistance-force relationship for a FSR sensor is illustrated in FIG. 3. The nature of this resistance-force relationship can be controlled through the design of the FSR material and the sensor, so that the sensor may have its sensitivity optimized for a specific force range. As seen in FIG. 3 the sensor produces a useless result for forces less then 70 grams, but has a very high sensitivity up to 200 grams. The sensitivity is reduced, but still significant in the 200 to 450 gram range, where the resistance level begins to flatten towards 3 kOhm.
Referring back to FIG. 2, also illustrated (at right) is a typical electrical connection, where an FSR sensor is typically connected as seen at 22 with a matching pull-down resistor, here a 3 kOhm resister. The sensor reading, Vout, is a function of the supply voltage, Vdc, and the resistance of the FSR sensor at a given applied force. This relationship is illustrated in FIG. 4, were it is demonstrated how the sensor reading (Vout) is increasing as the resistance in the FSR material decreases with the increase in the applied force. It should be noted that the force-Vout relationship would be very different for a piezo resistive sensor, where the relationship is linear from 0 gram force up through its operating range.
FIG. 5 illustrates the performance difference between different sample FSR sensors with the same design. It can be seen from FIG. 5 that the individual resistance value of one sensor sample differs significantly compared to a second sample, even within the same production batch. A third characteristic of the typical FSR sensor is that they are, unlike the Wheatstone bridge based piezo resistive force sensors, temperature dependent.
FIG. 6 illustrates how the resistance—force relationship changes for the same sensor measured under identical conditions, but at different temperature levels. These above-described characteristics make it difficult to design a differential-pressure touch pad assembly based on FSR technology. Nevertheless, the present inventors have devised a software compensation approach that allows the use of small and low-cost force sensing sensors, such as FSR sensors.
It would, therefore, be greatly advantageous to provide a force sensing technology which overcomes some of the above deficiencies of the prior art.